This invention relates to a circuit to protect batteries from over-current. When a non-rechargeable battery, which can only be discharged, or a rechargeable battery, which can be charged or discharged, is discharged (or charged for the case of a rechargeable battery) with a current in excess of that specified ("over-current"), the battery heats up, performance drops, and life-time is degraded.
As recited in Japanese Non-examined Patent Publication No. 63-158744 published Jul. 11, 1988, over-current protection circuits utilizing breakers and PTC devices (devices which increase resistance due to Joule heating) have been developed to prevent over-current discharge of batteries.
Unfortunately, an over-current protection circuit using breakers or PTC devices cannot prevent over-current discharge in a desirable manner. For example, in an over-current protection circuit using a PTC device, some time is required for the resistance of the PTC device to increase due to over-current. Therefore, during the time required for the resistance of the PTC device to increase, the battery has already discharged with excessive current. It is thus the first object of the present invention to overcome this drawback and protect a battery against over-current discharge and/or charge.
An over-current protection circuit, provided with elements such as semiconductor switches for quickly interrupting discharge when over-current is detected, can cut-off a switch as soon as battery discharge current above the specified value is detected.
This type of over-current protection circuit can immediately cut-off a switch to reliably suppress battery heating due to over-current. However, this type of circuit has the following problem. Namely, when the battery is connected to electrical equipment and the electrical equipment power switch is turned on, a starting surge of in-rush current greater than that for normal operation flows for an instant. Consequently, an over-current protection circuit, that immediately cuts off battery discharge when a current over the specified value is detected, will also cut-off due to starting surge. An over-current protection circuit with the over-current specification increased to avoid cut-off at the starting surge cannot safely protect the battery from over-current discharge. It is thus the second object of the present invention to provide an over-current protection circuit that can appropriately protect a battery from over-current without faulty operation caused by large starting surge currents.
An over-current protection circuit with semiconductor switches to quickly cut-off discharge when over-current is detected, can immediately cut-off a switch to reliably suppress battery heating when battery discharge current is detected above the specified value. For example, over-current discharge can be effectively prevented even in the case of a momentary short circuit. However, when the momentary battery short circuit and over-current condition is relieved, it is necessary for the cut-off switch to be returned to a condition allowing suitable discharge. It is thus the third object of the present invention to provide an over-current protection circuit that can reliably prevent battery over-current, as well as immediately return to a state allowing discharge after the over-current condition has been relieved.